Attention plays a crucial role in human performance by enabling us to selectively process relevant information, and attention deficit is a serious and prevalent neurological disorder. However, despite the significance of attention to human health, the neuronal basis of attention is poorly understood. Attention-related neuronal signals have been identified in many brain areas, in particular the prefrontal area and visual cortex, but it is unclear how these signals give rise to the behavioral improvements provided by attention. In most neurophysiological studies, attention-related signals are thought to give rise to behavioral improvements by enhancing the sensory processing of the attended stimulus. However, whether these signals in fact reflect enhanced sensory processing is unclear. In most studies, attention is directed to a location by cueing where the target is most likely to appear, resulting n more target detected and shorter reaction times at the cued location. While these improvements could be due to enhanced sensory processing, they could also arise through changes in the subject's decision criterion, which determines how much sensory evidence is required for reporting a target. Because the target is most likely to appear at the cued location, a decision criterion requiring less sensory evidence at the cued location would result in the same behavioral improvements as from enhanced sensory processing. Psychophysics studies have shown that changes in decision criterion could account for a large portion or all of the behavioral improvements of attention. Using methods that provide separate behavioral measures of the quality of sensory processing (sensitivity) and decision criterion, these studies show that attention often changes both sensitivity and the decision criterion and seldom modulates the two independently. However, neurophysiological studies of attention have not distinguished between sensitivity and decision criterion. This is a crucial issue. Given that decision criterion could account for a large portion of the behavioral improvements of attention, much of the attention-related signals attributed to sensitivity may in fact underlie changes in decision criterion. This project will determine whether attention-related neuronal signals can be parsed into two distinct components that separately correspond to changes in the subject's sensitivity and decision criterion. My specific aims are determine (1) whether changes in sensitivity and decision criterion are associated with separate neuronal signals in dorsolateral prefrontal cortex, an area involved in both attention and decision-making, and (2) whether the attention-related activity in area V4 of visual cortex, where the classic neuronal signals of attention were found, and the interaction between V4 and dorsolateral prefrontal cortex are differentially modulated by changes in sensitivity and decision criterion. The results of this project have the potential to greatly clarify our understanding of the neuronal mechanism of attention and define sub- modalities of attention that might be impaired to unequal extents in different disorders of attention.